Surfactant-Directed Synthesis of Mesoporous Pd Films with

Aug 25, 2015 - Palladium (Pd) films with perpendicularly aligned mesochannels are expected to provide fascinating electrocatalytic properties due to t...
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Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts Cuiling Li, Bo Jiang, Nobuyoshi Miyamoto, Jung Ho Kim, Victor Malgras, and Yusuke Yamauchi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06278 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015

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Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts Cuiling Li,1 Bo Jiang,1,2 Nobuyoshi Miyamoto,3 Jung Ho Kim,4 Victor Malgras,1 and Yusuke Yamauchi1,2* 1

2 3

4

World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan. Department of Life, Environment and Materials Science, Fukuoka Institute of Technology (FIT), 3-30-1 Wajiro-Higashi, Higashi, Fukuoka 811-0295, Japan Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia.

Abstract: Palladium (Pd) films with perpendicularly aligned mesochannels are expected to provide fascinating electrocatalytic properties due to their low diffusion resistance and the full utilization of their large surface area. There have been no studies on such mesoporous metal films, because of the difficulties in controlling both the vertical alignment of the molecular template and the crystal growth in the metallic pore walls. Here we report an effective approach for the synthesis of mesoporous Pd films with mesochannels perpendicularly aligned to the substrate by an elaborated electrochemical deposition. The films show a superior electrocatalytic activity by taking full advantage of the perpendicularly aligned mesochannels. ‘Surfactant assembly’ provides a general way for preparing ordered mesoporous materials.[1] Many mesoporous materials with different mesostructures and morphologies have been obtained by optimizing the synthetic conditions, even a same surfactant can result in different mesopore arrangements by changing the concentration. Among various morphologies, mesoporous films have sparked many keen interests in a wide range of potential applications (e.g., catalysis, energy storage, photonic therapy) due to their high accessible active surface, superior structural stability and unique functions.[2] In general, the surfactant micelles tend to orient parallel to the substrate in order to reduce the surface energy at the interface, resulting in mesochannels inevitably oriented parallel to the substrate and making the internal mesopores inaccessible. Considering practical applications, a precise control over the mesopore orientation is a critical issue to overcome in order to fully exploit the internal pores. Perpendicularly oriented mesochannels in films are an ideal structure which allows the diffusion of reagents from the exterior region to the inner parts, thus favoring the high utilization efficiency brought by the mesopores.[3] Although several strategies employing high magnetic field,[4] electro-assisted self-assembly,[5] air flow,[6] confined effect,[7] and optimal evaporation-induced self-assembly (EISA)[8] have been previously employed for achieving vertically oriented mesochannels, the compositions in pore walls have been mostly limited to silica, carbons, organosilica, titania, and alumina. To the best of our knowledge, there have been no reports on metal films with perpendicular mesochannels. Mesoporous metal films possess a high surface area and their fairly stable metallic frameworks provide a large number of catalytically active sites, thereby exhibiting fascinating catalytic properties.[9] Palladium (Pd) has been well known as an exceptional catalyst for a rich variety of applications, including oxidation of

fuels, carbon-carbon coupling reactions and hydrogen storage/detections.[10] Therefore, mesoporous Pd films with perpendicular mesochannels can bring out many superior electrochemical applications which cannot be achieved by other previously reported mesoporous films.[4-8] Although several approaches using lyotropic liquid crystals, surfactant micelles, hard-templates have been utilized for the preparation of mesoporous metals,[11] mesoporous metal films with perpendicular mesochannels have never been achieved.

Figure 1. (a, b) SEM images of the top-surface of the obtained Pd films. (c, d, e) Cross-sectional TEM images of the obtained Pd films [(c) Bright-field TEM image, (d) high resolution TEM image, and (e) HAADF-STEM image].

Herein, we report a sophisticated amperometric approach which reduces the Pd species decorated on the surface of the micelles leading to the formation of continuous Pd films with vertically aligned mesochannels. The resulting mesoporous Pd films distinctly fully exploit the ultra-high surface area ensued from the mesoporous architecture and clearly exhibit linearly

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increasing electrocatalytic activities with increasing the film thickness. In this work, a solution containing cationic surfactant, cetylmethylammonium chloride (C16TAC), and Na2PdCl4 was firstly prepared and used as electrolyte. Then, a conductive Au substrate was employed as a working electrode for the electrodeposition of Pd at a constant potential for 600 s under optimized conditions (See Supplementary Information for details.). After completely removing the surfactants, Pd films with perpendicular mesochannels were obtained.

Figure 2 (a, b) 2D GI-SAXS patterns of the obtained Pd films with perpendicular mesochannels after different deposition times of (a) 600 s and (b) 3600 s. (c) Intensity profile collected in the in-plane direction against q value. (d) Wide-angle XRD patterns of bare Au substrate and the obtained Pd film with perpendicular mesochannels prepared for a deposition time of 600 s.

The top surface and cross-section of the obtained films were studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM). From the top-view, uniformly sized mesopores are distinctly distributed over the entire area of the film without any defects, cracks, or bumps (Figs. 1a-b and Fig. S1). The size of the mesopores is homogeneously distributed with a predominant pore size of 2.1±0.2 nm separated by pore walls of 3.3±0.2 nm. Various mesoporous materials have been synthesized using cetylmethylammonium chloride/bromide (C16TAC/C16TAB) with the same hydrocarbon chain length as the pore directing agents. The pore size obtained in this study is well consistent with the ones reported previously (Table S1). The cross-sectional TEM image and the high-angle annular dark-field scanning TEM (HAADF-STEM) image show that cylindrical channels are distributed perpendicular to the substrate (Fig. 1c-e, and Fig. S2). In the high-resolution TEM image, it can be observed that the framework is highly crystalized without any amorphous domains, although the lattice fringes can be slightly distorted at certain locations (Fig. 1d and Fig. S3). The clear lattice fringes with a constant d spacing of 0.225 nm can be ascribed to the (111) lattice planes of fcc Pd crystals. A uniform thickness of 24 nm was distinctly grown on the Au surface (Fig. 1c and Fig. S4). Although the long range in-plane ordering of the mesopores seems limited (Fig. S5), the mesochannels are all perpendicularly aligned to the substrate.

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2D grazing-incidence small-angle X-ray scattering (GISAXS) was employed to further confirm the orientation of mesochannels (Fig. 2). The two intense spots correspond to the inplane periodicity of the obtained films (Fig. 2a). The absence of out-of-plane periodicity can rule out any possibility of other threedimensional (3D) mesostructures.[12] The intensity profile collected in the in-plane direction plotted against q value (Fig. 2c) reveals a peak centered at q=1.17 nm-1 corresponding to a pore-topore d-spacing of 5.4 nm. Considering the wall thickness (~3.3 nm) and the pore size (~2.1 nm), the GI-SAXS data are in good agreement with the SEM results (Fig. S1 and Fig. S5). Conceptually different from the evaporation-induced selfassembly (EISA) method which is generally utilized for preparing mesoporous films,[13] the electrodeposition approach used in this work allows an easy control over the film thickness by changing the deposition time. From cross-sectional images study, the film thickness was determined to increase linearly with the deposition time, with an average growth rate of 0.076 nm s-1 (Fig. S6). The 2D GI-SAXS spots are unaffected by increasing the deposition time (Fig. 2b-c), suggesting that any film thicknesses can be obtained by choosing a suitable deposition time without any interfering with the perpendicularly aligned mesoporous structures. No changes in the vertically aligned mesochannels are noticed even after increasing the deposition time up to 7200 s (leading to a film thickness increases of 540 nm) (Fig. S7). From energy dispersive X-ray spectroscopy (EDS) analysis, it can be observed that the obtained mesoporous films consist of only Pd (not shown). The crystalline structure of Pd was examined by wide-angle X-ray diffraction (XRD). The XRD patterns obtained from the electro-deposited films only gave two diffraction peaks assigned to the (111) and (222) diffraction planes of face-centered cubic (fcc) Pd (JCPDS No. 87-643) (Fig. 2d), indicating that the Pd crystals have a preferential orientation according to the (111) plane parallel to the substrate on the seed layer (Au). The Pd crystal growth along the [111] direction is consistent with TEM result (Fig. 1d). The surfactant assembly, which is usually concentrationdependent, is critical to determine the final mesoporous structure. As the surfactant concentration increases, the surfactant assembly changes from free surfactants to spherical and cylindrical micelles.[14] In our system, the concentration of C16TAC is 43 mM, which is higher than its critical micelle concentration (CMC, 16 mM), suggesting the formation of spherical micelles in the electrolyte (Fig. S8). It has been known that the strongly charged anionic inorganic species interact with the cationic surfactants.[15] The interactions between the C16TAC micelles and the Pd precursors in the electrolyte were studied by UV-vis absorption (Fig. S9). The [PdCl4]2- species exhibit two absorption maxima at 210 and 235 nm.[16a] After being mixed with C16TAC micelle solution, a significant change is observed with the newly formed absorption bands at 226 nm and 285 nm, which are attributed to the interactions between the anionic [PdCl4]2- species and the cationic [C16TA]+ surfactants.[16b] From the small-angle X-ray scattering (SAXS) measurement of the electrolyte after the addition of [PdCl4]2- species, it is found that the spherical micelles are well retained (Fig. S8). It is probable that the spherical micelles decorated with [PdCl4]2- species approach the working electrode when an exterior potential is applied. The Pd species are initially reduced on the surface of the working electrode. Due to its reduction, each [PdCl4]2- releases four chloride ions, which then probably bind to the surfactants more strongly and induce the sphericalto-rod transition. The formed rod-like micelles can be well oriented perpendicular on its surface due to its stronger affinity with the hydrophobic core than the hydrophilic shell of the micelles, although cylinder micelles and lamellar phases generally have a strong tendency to align parallel to the substrate surfaces due to their lower energy.[17] The subsequent Pd deposition and reassembly of surfactant molecules lead to a continuous growth

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along the longitudinal direction of the cylindrical micelles perpendicularly orientated on the substrate (Fig. S10).

Figure 3 (a) CVs in 0.5 M H2SO4 of the obtained Pd films with perpendicular mesochennels with various film thicknesses (The deposition times are 300 s, 600 s, 1000 s, 1500 s, 2000 s, respectively). (b) Relationship between the ECSAs and the deposition times. The mesoporous Pd film (i) is compared to a nonporous Pd film (ii). (c) CV curves and (d) potential dependent steady-state current density for formic acid oxidation catalyzed by a Pd film with perpendicular mesochannels (i), a nonporous Pd film (ii), and a commercially available PdB catalyst (iii). The formic acid oxidation reaction is performed in 0.5 M H2SO4 containing 0.5 M HCOOH.

The retention of the ordered mesophases formed by the surfactant assembly is critical for designing the pore walls. In the case of metals, their crystallized walls are formed simultaneously with the reduction of metal species, unlike post-treated crystallized/graphitized pore walls. In the present system, the crystal growth can be controlled by tuning the reduction conditions. Especially, electrochemical reduction is useful for strict control of the growth speed and the crystallinity of the deposited metals. Various constant potentials in the range of -0.5 V and 0.3 V (vs. Ag/AgCl) were employed for the electrodeposition of Pd films. The mesoporous structures in the obtained films were studied by SEM and 2D GI-SAXS (Fig. S11). During the formation of mesoporous metals, the reduced metal atoms are primarily formed at the hydrophilic region of the micelles where they agglomerate and coalesce until a thick wall is obtained, giving the structure enough mechanical stability. A slow reduction reaction generally allows the surfactant templates to be distorted so large crystals can grow.[18a] When a high potential is applied (e.g., 0.2 V, and 0.3 V (vs. Ag/AgCl) in the present work), the reduction of the Pd precursors becomes slower and the mesoporous structures cannot be achieved (Fig. S11a-b). Therefore, an appropriate reduction rate is necessary for the preparation of mesoporous metals. Wellordered mesoporous structures can be obtained when a potential in the range of 0.1 V and -0.1 V (vs. Ag/AgCl) is applied, as evidenced by the top-surface SEM images and the 2D GI-SAXS profiles (Fig. S11c-e). When the reduction rate is further increased, the initially formed Pd crystals are larger and end up exceeding the size of the hydrophilic volume of the micelles, destroying the ordering of the micelle assembly (Fig. S11f-h).[18b] Through the above results, a moderate potential (i.e., -0.1 V, 0 V, and 0.1 V (vs. Ag/AgCl)) is essential to control the Pd reduction rate without interfering with the micelle assembly. The surfactant concentration is also critical (Fig. S12). When it is lower than CMC, no porous constructions can be observed, while highly

increased concentration (five or ten times higher than the optimal concentration) leads to the formation of perpendicular mesochannels, but with a slightly reduced periodicity. Motived by the fact that the perpendicularly aligned mesochannels have naturally a superior mass transportation than other architectures, electrochemical measurements were carried out. Here, cyclic voltammograms (CVs) in a 0.5 M H2SO4 solution were firstly performed on the obtained Pd films with various thicknesses. A noticeable current increase in the hydrogen and oxygen adsorption/desorption regions was observed (Fig. 3a). The electrochemical surface area (ECSA) was calculated by using the method proposed by Woods (see Supplementary Information for details). The ECSA was determined to increase linearly with the film thickness (i.e., deposition time), unlike the nonporous Pd films prepared without surfactants (Fig. 3b and Fig. S13). The volume-normalized ECSA of the Pd film with perpendicular mesochannels is calculated to be 737 m2·cm-3, which is much higher than that of the mesoporous Pt films prepared from lyotropic liquid crystals (at most 460 m2·cm-3).[19] Thus, it is found that the perpendicularly oriented mesochannels are able to make the whole inner porous structure accessible. To gain deeper insights regarding the electrocatalytic properties of the Pd films with perpendicular mesochannels, formic acid oxidation reaction was selected as a model reaction. The mesoporous Pd film was evaluated and compared to a nonporous Pd film and a commercially available Pd black (PdB) catalyst. The typical CV curves for HCOOH oxidation catalyzed by different catalysts are shown in Fig. 3c. It can be clearly observed that the Pd film with perpendicular mesochannels shows a higher current density of 934.6 mA mg-1_Pd, which is around 2.7 and 1.9 times higher than those of the nonporous Pd film (343.0 mA mg-1_Pd) and the PdB (485.8 mA mg-1_Pd), respectively. In addition, the steadystate current densities obtained on the three different catalysts are also compared (Fig. 3d). At a given oxidation current density, as indicated by dashed line in Fig. 3d, the corresponding potential on the mesoporous Pd film is much lower than that on the nonporous Pd film and the PdB. The potential on the mesoporous Pd film is negatively shifted by ~80 mV and ~50 mV, compared with the nonporous Pd film and PdB catalyst, respectively. Thus, the mesoporous Pd film with perpendicular mesochannels exhibits a strongly enhanced catalytic activity (i.e., enhanced Pd utilization efficiency). This is probably attributed to a higher active surface caused by the accessible mesochannels perpendicularly aligned to the substrate. In this work, we demonstrated a novel approach for the successful preparation of Pd films with perpendicular mesochannels. The pore size is uniform and straight mesochannels are evenly distributed in the entire area of the obtained films regardless the substrate sizes. A highly enhanced electrocatalytic performance toward formic acid oxidation was observed. The present approach is not only highly reliable and effective for creating vertically aligned mesoporous electrocatalysts, but also paves the way for other metals and alloys towards achieving a superior catalytic activity. Supporting Information Experimental details and additional data are available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected] References [1] (a) Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D.; Kim, J. M.; Stucky, G.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449. (b) Suteewong, T.; Sai, H.; Cohen, R.; Wang, S.; Bradbury, M.; Baird, B.; Gruner, S. M.; Wiesner, U. J. Am. Chem. Soc. 2011, 133, 172. (c) Crepaldi, E. L.; Soler-Illia, G. J. A. A.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez,

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